Apparatus and method of producing semiconductor rods by pulling the same from a melt



Dec. 26, 1967 I w. KELLER 3,360,405

APPARATUS AND METHOD OF PRODUCING SEMICONDUCTOR Filed April 29, 1965' RODS BY PULLING THE SAME FROM A MELT 2 Sheets-Sheet 1 Dec. 26, 1967 w. KELLER 3,360,405 APPARATUS AND METHOD OF PRODUCING SEMICONDUCTOR I 4 RODS BY PULLING THE SAME FROM A MELT Filed April 29, 1965 2 Sheets$heet 2 -FIG..6

United States Patent s 18 Claims. (Cl. 14s 1.s

My invention relates to apparatus and method for producing semiconductor rods by pulling the same from a melt of semiconductor material.

This application is a continuation-in-part of my copending application, Ser. No. 351,032, filed Mar. 11, 1964, now abandoned.

Monocrystalline semiconductor rods have been produced in the past by pulling them from a melt according to the Czochralski method and by crucible-free zone melting according to the method of Theuerer. More recently, the so-called Podest process has become known (see the article by Dash on page 363 of Growth and Perfection of Crystals, edited by Doremus, Roberts and Turnbull, published by John Wiley & Sons, Inc., New York, and Chapman and Hall, Ltd., London, 1958. A drop-shaped melt is produced on a slotted or split semiconductor rod, for example by means of induction heating, and a monocrystal is drawn out of this melt after immersing a monocrystalline seed therein.

In the method of pulling monocrystals from a melt in a crucible, the diameter of the growing semiconductor material is controlled by regulating the temperature of the melt and the speed at which the pulling takes place or both. By using a monocrystalline seed, semiconductor monocrystals of larger diameters can be produced in this manner. This method, however, has a disadvantage in that impurities, such as oxygen, can diffuse into the melt from the heated crucible wall. With materials that melt at high temperatures, such as silicon for example, further difficulties arise due to the fact that the crucible wall becomes plastically deformable at those high temperatures. With the crucible-free zone melting method, monocrystals with a diameter of more than 25 mm. can be produced only with great difficulty, and it is almost impossible to produce monocrystals of more than 35 mm. diameter.

It is accordingly an object of my invention to provide an apparatus and method of producing semiconductor rods which avoids the disadvantages of the previously known apparatuses and methods and which permits the production of semiconductor rods of relatively large diameters with ease.

It is another object of my invention to provide an apparatus and method of producing semiconductor rods in which the semiconductor rods that are grown are of relatively large diameter and are monocrystalline.

It is an additional object of my invention to provide an apparatus and method of producing semiconductor rods wherein contamination by diffusion from the crucible walls is prevented by making the crucible walls proper of the same highly purified semiconductor material.

It is a concomitant object of my invention to provide an apparatus and method of producing semiconductor bodies wherein the body that is grown is practically undisturbed while it is being heated and the growing crystal accordingly has exceptionally few dislocations, a feature which is known to be particularly important in the production of monocrystalline semiconductor rods that are to be used as electronic components.

In accordance with an aspect of my invention, I provide an apparatus and method of producing semiconductor rods by pulling the same from a melt in which a 3,360,405 Patented Dec. 26, 1967 rod component, i.e. a seed crystal particularly, is immersed in a melt and is enlarged as it is pulled out. A substantially cylindrical semiconductor carrier element is provided, having a longitudinal axis extending in a vertical direction, and the semiconductor carrier element is rotated continuously about this axis. The semiconductor body is heated from above by means of a heating device disposed at one side of the rotational axis and extending up to about the center of the body cross section, and the rod portion beyond the heated portion of the melt is pulled out of the latter.

In accordance with another aspect of my invention and in order to avoid imperfections of the growing semiconductor monocrystal, it is pulled from the melt outside of the field of the induction heating coil in the case where heating is carried out by induction.

In accordance with yet another aspect of my invention, I provide apparatus and method of producing a monocrystalline semiconductor body in tape or web form by inserting a dendritic seed crystal into the melt and then pulling it therefrom.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims.

While the invention has been illustrated and described as apparatus and method of producing semiconductor rods by drawing them from a melt, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. Such adaptations should, and are intended to be comprehended within the meaning and range of equivalents of the claims herein.

The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

FIG. 1 is a diagrammatic cross section of a vacuum chamber in which the method of this invention can be carried out;

FIG. 2 is an enlarged perspective view of the central components shown in the vacuum chamber of FIG. 1;

FIGS. 3 and 4 are top plan views of modifications of the components shown in FIG. 2; and

FIG. 5 is a perspective view of an additional modification of the components shown in FIG. 2.

FIGS. 6 and 7 are a perspective view and a sectional view, respectively, of an additional embodiment of the invention.

Referring now to the drawings and particularly to FIG. 1 thereof, there is shown a vacuum chamber comprising a box-type housing 2 provided with a viewing glass or window 3 through which the method carried out in accordance with my invention inside the chamber can be observed. Instead of a vacuum, a protective gas can be supplied to the housing 2 in order to carry out the method of this invention. The chamber can be evacuated or supplied with protective gas through the connecting tube 4. A thick rod 5 as well as a slender rod 6, both consisting either of silicon or germanium, are located inside the chamber, the slender rod 6 being produced or grown from and carried by the thick rod 5. A melt 7 is located between the rods 5 and 6 and is formed by heating and melting the top of the rod 5 with an induction coil 8, but can also be formed similarly by radiation heating or electron radiation. The induction coil 8 is secured to a support 9 of suitable insulating material which does not melt or vaporize at the employed temperatures. The support 9 extends outwardly from the chamber 2 through a vacuumproof or protective-gas-proof fitting 10, as the case may be, located in an opening at the bottom wall of the chamber 2. The support 9 has at least one longitudinal bore through which the electrical leads to the heating coil 8 and a coolant supply and discharge means for cooling the heating coil extend. The two-headed arrow 11 indicates that the heating coil 8 and the support 9 are displaceable from outside the vacuum chamber in a vertical direction as viewed in FIG. 1.

The thick rod is supported by a lower holder 12 that is secured at an end of a guide rod 13 which is also led to the outside through a vacuum-proof or protective-gasproof fitting 14, as the case may be, also mounted in an opening provided in the base wall of the chamber. The guide rod 13 can also be actuated from the outside for displacement in the vertical direction as viewed in FIG. 1 as well as for turning about its axis as indicated by the pertient two-headed and curved arrows. The slender rod 6 is held in a similar manner as the thick rod 5 in an upper holder 15 which is secured at the end of a shaft 16. The shaft 16 also passes through a vacuum or protectivegas-proof fitting 17, as the case may be, mounted in an opening in the top wall of the chamber and is also displaceable in a vertical direction from the outside as viewed in FIG. 1, as well as rotatable about is own axis as shown by the related arrows.

In FIG. 2, portions of the thick rod and the slender rod are shown in an enlarged view as engaging one another in a melt. As is apparent from the drawing, the slender rod portion 6 lies remote from the elfects of the heating coil 8 and is therefore able to grow completely undisturbed thereby. The heating coil 8 produces a heating effect on the portion of the end surface of the thicker rod 5 which lies beneath it, causing it to melt. By continuous rotation of the thicker rod 5 about its longitudinal axis, every portion of the outer end surface of the thick rod is subjected to the heating effect of the coil 8. On the other hand, the slender rod 6 which is located eccentrically with respect to the longitudinal axis of the thick rod 5 is not affected by the heating device. It is understood, of course, that instead of the single winding of the illustrated heating coil 8, a heating coil with several windings, such as two or three, for example, can be employed. The slender rod 6 is advantageously rotated about its own axis so as to ensure symmetrical crystalline growth thereof. The thick rod 5 is suitably a cylindrical semiconductor body of silicon or germanium. Slight variations in the diameter of the cylindrical rod 5 are of no significance. Also slight variations in the shape of the cylinder, for example where a slight taper of the surface gives it a somewhat conical appearance, are not harmful for carrying out the method of this invention. Naturally, it is particularly advantageous when the rod is substantially a geometrically perfect cylinder.

In FIG. 3 there is illustrated a slightly modified heating coil 8a in which one of the leads is shown as being drawn otf straight from the heating loop rather than having a bend as in the embodiment of FIG. 2.

In FIG. 4 there is shown another heating coil 8b which dilfers from that of FIGS. 2 and 3. The induction heating coil 8b of FIG. 4 has the form of a segment of a circle. The point of the circular segment is located at the rotational axis of the cylindrical thick rod 5. Such a shape of the induction heating coil produces a fairly uniform depth penetration of the applied heat over the entire surface of the melt 7 as the rod 5 is rotated.

The entire upper surface of the thick rod 5 is advantageously not heated but rather, a predetermined margin located between the arc-shaped portion of the coil 8b and the edge of the rod 5 remains unheated, which prevents dripping of the melt from the upper surface of the rod.

It is advisable to supply the melt continuously with new semiconductor material from an outside source and thereby prevent the cylindrical semiconductor body 5 from oeing entirely consumed. In such a case the cylindrical semiconductor body 5 can be kept relatively short in length as shown in FIG. 5. The semiconductor material which is supplied to the melt is preferably in the form of another semiconductor rod and is also preferably introduced in the vicinity of the heater so that it will melt as soon as possible. In the case where heating is effected by an induction-type heating coil as shown in FIG. 5, the additional semiconductor material in the form of the rod 18 is introduced into the melt through the circular turn of a heating coil 80. The cylindrical semiconductor body 5 in such a case is not necessarily but preferably surrounded by a graphite crucible 19. In order to reduce the heating capacity or necessary heating power for the induction heating coil 8c, the graphite crucible 19 is preheated; for example in the case where silicon is the semiconductor, the cylindrical semiconductor body and crucible are preheated to about 1200 C. Other types of preheating can also be provided, for example by means of an induction heating coil which surrounds the thick rod 5 in the vicinity of the melt 7.

It is of course also contemplated within the scope of my invention to apply heat to the top of the thick rod 5 on one side of the vertical axis and to rotate the rod 5 to produce the melt 7 by other heating means than the induction coil 8, 8a, 8b, 8c. As aforementioned, such applications can be made by radiation heating, electron radiation heating, or the like.

In the embodiment of FIGS. 6 and 7, semiconductor material in the form of a powder 23 is placed in a crucible 22 consisting, for example, of graphite, quartz or another suitable material such as the semiconductor material proper that is being processed. The crucible 22 is preferably circular and rotatably mounted and is rotated during the operation about its central axis. Part of the semiconductor material 23 is melted by means of a heating device. The heating device can consist of a heating coil 24 as shown, for example in FIGS. 6 and 7, which is electrically connected to a high frequency generator (not shown) that energizes the heating coil with an alternating current having a frequency of about 0.5 to 5 megacycles. The heating device 24 is located above the crucible and is fixed with respect to the rotary axis of the crucible. The heating device 24 is furthermore located at a position intermediate to and spaced from the center of rotation of the crucible 22 and the periphery thereof. As the crucible is rotated, a ring-shaped zone 25 of the semiconductor material 23 passes continuously beneath the heating device 24. Due to the heating effect of the heating device 24, the circular ring-shaped Zone 25 is melted and forms a ring-shaped melt. If desired, an inversion of the movements can be provided by maintaining the crucible stationary and moving the heating device or the zone of application of the heating device in a circular or ring-shaped path over the semiconductor material 23.

Instead of the heating coil 24, another heating source, for example in the form of electron beams or heat rays, as aforementioned, can be provided whereby, for example when employing a stationary crucible 22 containing semiconductor material 23, heating of a ringshaped portion of the semiconductor material can be effected by suitable movement of the source of the beams or rays. If desired, other heating devices can also be provided, for example, a heating system or device located in the Wall of the crucible for directly heating the semiconductor material in the crucible. It is advantageous to preheat the semiconductor material to a specific temperature just below the melting point of the semiconductor material by means of a heating device which heats the crucible 22 or all of the semiconductor material 23 in the crucible and, with an additional heating device, to supply to the ring-shaped zone the additional heat necessary for raising the temperature of the zone so as to melt the semiconductor material. It is also advantageous to provide a plurality of heating devices distributed circumferentially about the annular zone 25 for heating the zone simultaneously or sequentially.

At a location spaced from the heating device 24, a seed crystal 26 is dipped in the melt 25 and then pulled out of the melt. By continuous growth of the semiconductor material, this seed crystal 26 is enlarged to produce a desired monocrystal of the semiconductor material.

The method according to my invention can be used especially for producing dendritically grown semiconductor material since the growing crystal is uninfiuenced by the heating device. When a dendritic seed crystal, i.e. a monocrystal with a twinned surface or twinning plane is thus dipped into the melt, the semiconductor material growing thereon generally displays dendritic growth. Consequently, the method of my invention permits growth of dendritic silicon crystals in the form of a tape as shown in FIG. 6.

A further advantage of the method of my invention is that the temperature at the location where the seed crystal is dipped into the melt can be accurately regulated. Provision can be made, for example, that the melt at the location at which the heat is applied thereto has a temperature for the most part which is 10 to 20 above the melting point of the particular semiconductor material used. Since the melt is continuously moved past the location or locations of direct heating, its temperature is diminished with the distance removed from the heating location or locations. Consequently, in the circumferential direction of the ring-shaped melt, a location can be found having virtually any desired temperature. It is important in this respect that the heating action be so adjustable that the melt, when passing into the location of direct heating, has already been supercooled so that supercooled locations already exist beforehand. Particularly with dendritic growth it is necessary in most cases to pull the monocrystal out of a supercooled melt.

The method of my invention can in addition also be employed advantageously for pulling semiconductor tapes which are overgrown on both sides with dendrites (socalled webs) out of the supercooled portion of the melt. For this purpose, a dendritic seed crystal is dipped into the supercooled melt in such a way that the pulling direction, which lies perpendicular to the surface of the melt,

corresponds to the 2 ll-direction or an equivalent direction. When the seed crystal is pulled swiftly, a button forms on the seed dendrites from which there grows outwardly a monocrystalline band of about 5 to mm. width and 50 to 200 1. thickness laterally defined by two dendrite strings.

The embodiment of FIG. 6 is shown in cross section in FIG. 7 of the drawing with an additional heating device 24' at a second location of the annular melt 25. The semiconductor material 23 is shown as a powder. Between the melt 5 and the semiconductor powder 3 there is located a trough-shaped annular zone 27 consisting of semiconductor powder that has been melted together or sintered together. This, trough-shaped zone 27 is automatically formed when the melt is produced.

When carrying out the method of my invention, the new semiconductor material is continuously supplied preferably to the melt 25 so that the location of the melt in the surrounding semiconductor material 23 in the crucible 22 is not changed and this surrounding semiconductor material is not used up. Naturally, one can also provide for the consumption of the surrounding semiconductor material if desired. However, the control and regulation of the temperature and the other conditions for the method are thereby rendered more difiicult. Just so much material is expediently supplied to the melting zone 25 as is drawn off therefrom by the growth of the monocrystal. For this purpose, a semiconductor rod 28 held above the crucible 22 is fed in to the melt. The heating device 24 suitably serves, as shown in FIGS. 6 and 7, to supply the new semiconductor material in fluid form.

It is also further possible to supply the semiconductor material in another form, for example in chunks or pieces or in powder form. For the purpose of more accurate regulation or control of the temperature of the melt, a temperature measuring sensor can be provided at a location just beyond the point of introduction of the new semiconductor material in the direction of rotation which can aid in controlling a heating device placed in a location further along the direction of rotation of the melt. If necessary, provision can be made for suitably correcting the temperature by means of additional temperaturesensitive devices which are serially connected. The ternperature of the melt at the location at which the seed crystal is pulled, or the degree of supercooling at this location can be adjusted by controlling the rotary speed of the melt and by changing the position of this location.

In accordance with the embodiment of FIGS. 6 and 7, the melt is produced in a pulverulent bed of the semiconductor material. Naturally, the semiconductor material can also be in chunks or in solid form, preferably in the form of coarse pieces which are similar to one another in cross-sectional shape. Thus, for example, the holder for the melt can be assembled of hexagonal rod portions which are formed by monocrystalline growth of semiconductor material from the gas phase. Rods of this type have a cross section with the shape of a regular hexagon so that rod portions of this type can be assembled in honeycomb fashion. In this case, if necessary, a surrounding and retaining crucible can be completely dispensed with. The entire embodiment shown in FIGS. 6 and 7 is advantageously surrounded with an evacuated vessel or a vessel filled with a protective gas as shown in FIG. 1.

I claim.

1. Method of producing a monocrystalline semiconductor body which comprises horizontally rotating a melt of semiconductor material simultaneously with a carrier of the same material underlying and supporting the melt; applying heat at a radially extending area overlying the rotating melt so as to heat the melt as it passes beneath the area to at least the melting temperature of the material; and pulling with the aid of a crystal seed a mono crystalline semiconductor body from a portion of the melt distantly located from the applied heating area.

2. Method of producing a monocrystalline semiconductor body which comprises horizontally rotating a melt of semiconductor material simultaneously with a carrier of the same material underlying and supporting the melt; applying heat at a radially extending area overlying the rotating melt so as to heat the melt as its passes beneath the area to at least the melting temperature of the material; pulling with the aid of a crystal seed a monocrystalline semiconductor body from a portion of the melt distantly located from the applied heating area; and 'supplying additional semiconductor material in solid form to the melt to replenish the material pulled from the melt.

3. Method of producing a monocrystalline semiconductor rod which comprises horizontally rotating a substantially cylindrical block of semiconductor material simultaneously with a melt of the same material carried by the block; applying heat at a radially extending area overlying the rotating melt so as to heat the melt as it passes beneath the area to at least the melting temperature of the material; and pulling with the aid of a crystal seed a monocrystalline semiconductor rod from a portion of the melt distantly located from the heating area.

4. Method of producing a monocrystalline semiconductor rod which comprises horizontally rotating a substantially cylindrical block of semiconductor material simultaneously with a melt consisting of the same material which is supported by the block; applying heat at a radially extending area overlying the rotating melt so as to heat the melt as it passes beneath the area to at least the melting temperature of the material; pulling with the aid of a crystal seed a monocrystalline semiconductor rod from a portion of the melt distantly located from said heating area; and introducing semiconductor material in rod form to said melt from above the same to replace the material pulled from said melt.

5. Method of producing a monocrystalline semiconductor rod which comprises horizontally rotating a substantially cylindrical carrier of semiconductor material simultaneously with a melt consisting of the same material and supported on the carrier; applying heat at a radially extending area overlying the rotating melt .so as to heat the melt as it passes beneath the area to at least the melting temperature of the material; pulling with the aid of a crystal seed a monocrystalline semiconductor rod from a portion of the melt distantly located from said heating area; and introducing semiconductor material in solid form to the melt through the heating area from above the melt so as to replace the material pulled from the melt.

6. Method of producing a monocrystalline semiconductor rod which comprises horizontally rotating a substantially cylindrical carrier of semiconductor material simultaneously with a melt consisting of the same material overlying and supported by the carrier; applying heat at a fixed area overlying the rotating melt and extending radially to a marginal area adjacent the periphery of the cylindrical carrier so as to heat all of the melt encircled by the marginal area as it passes beneath the heating area to at least the melting temperature of the material; and pulling with the aid of a crystal seed a monocrystalline semiconductor rod from a portion of the melt distantly located from said heating area.

7. Apparatus for producing a monocrystalline semiconductor body comprising a semiconductor carrier, means for rotating said semiconductor carrier about a vertical axis, means for applying heat from a position above said carrier, onto an upper face of said carrier over a fixed area extending radially outwardly from the vicinity of said vertical axis, said upper face of said carrier being rotatable through said area to form a melt of semiconductor material around said vertical axis; and means for pulling a crystal seed and consequent semiconductor growth thereon from said melt at a location distant from said heating area.

8. Apparatus according to claim 7 including means for supplying semiconductor material to said melt from above the same so as to replace the material pulled therefrom.

9. Apparatus according to claim 8 wherein said heatapplying means comprises an induction heating coil located above and spaced from said melt, said coil having the shape of a sector of a circle with its vertex directed toward said vertical axis, the area defined by said sectorshaped coil corresponding substantially to said heat-applying area.

10. Apparatus according to claim 9 including means for adjusting the spacing between said induction heating coil and said melt.

11. Method of producing a monocrystalline semiconductor tape or web which comprises horizontally rotating a ring-shaped melt of semiconductor material simultaneously with a carrier of the same material underlying and supporting the melt; applying heat at a location intermediate the rotational center of the carrier and a peripheral edge thereof and overlying the rotating melt so as toheat the melt as it passes beneath the location to at least the melting temperature of the material; and pulling with the aid of a dendritic seed crystal a monocrystalline semiconductor tape from a location of the melt distant from the applied heating location.

12. Method according to claim 11 including the step of continuously supplying semiconductor material to the melt in sufiicient quantity to replace the semiconductor material pulled from the melt whereby the material of the carrier is not consumed.

13. Method according to claim 12 wherein the semiconductor material is applied to the melt in fluid form.

14. Method according to claim 11 wherein the dendritic seed crystal is pulled from the melt at a supercooled location thereof.

15. Apparatus for producing a monocrystalline semiconductor tape or web comprising a carrier of semiconductor material having a peripheral edge; means for rotating said carrier about a vertical axis; means for applying heat from a position above said carrier onto an upper face of said carrier at a location thereof intermediate said vertical axis and the edge of said carrier, said upper face of said carrier being rotatable through said area to form a ring-shaped melt of semiconductor material around said vertical axis; and means for pulling a dendritic crystal seed and consequent dendritic growth of monocrystalline semiconductor material thereon from said melt at a location thereof distant from said heating location.

16. Apparatus according to claim 15 wherein the semiconductor material of said carrier is in powder form and is contained in a rotatable crucible.

17. Apparatus according to claim 15 wherein the semiconductor material of said carrier is in particulate form and is contained in a rotatable crucible consisting of similar semiconductor material.

18. Apparatus according to claim 15 wherein said heat applying means comprises a plurality of heating devices spaced from each other and located circumferentially about said vertical axis at substantially the same radial distance between said vertical axis and the peripheral edge of said carrier.

References Cited UNITED STATES PATENTS 2,809,136 8/1957 Mortimer 148-l.6 2,858,199 10/1958 Larson l48l.6 2,962,363 11/1960 Martin l48l.6 2,979,386 4/1961 Shockley et al. l48l.6 3,044,967 7/1962 Sterling et a1. l48l.6 3,160,497 12/1964 Loung l48l.6 3,228,753 1/1966 Larsen l48l.6

DAVID L. RECK, Primary Examiner.

N. F. MARKVA, Assistant Examiner. 

1. METHOD OF PRODUCING A MONOCRYSTALLINE SEMICONDUCTOR BODY WHICH COMPRISES HORIZONTALLY ROTATING A MELT OF SEMICONDUCTOR MATERIAL SIMULTANEOUSLY WITH A CARRIER OF THE SAME MATERIAL UNDERLYING AND SUPPORTING THE MELT; APPLYING HEAT AT A RADIALLY EXTENDING AREA OVERLYING THE ROTATING MELT SO AS TO HEAT THE MELT AS IT PASSES BENEATH THE AREA AT LEAST THE MELTING TEMPERATURE OF THE MATERIAL; AND PULLING WITH THE AID OF A CRYSTAL SEED A MONO- 